Disclosed are a fine grain rare earth alloy casting piece, a preparation method and a rotary cooling roller device. The alloy casting piece has a roller-contacting surface and a free surface, and comprises grains with an R2Fe14B compound as the main phase, on the cross section along the temperature gradient, with the grains comprising non-columnar grains and columnar grains, wherein the proportion of the non-columnar grains with an aspect ratio of 0.3-2 with respect to the area percentage of the grains is ≥60%, and with respect to the number percentage of the grains is ≥75%; the proportion of the columnar grains with an aspect ratio ≥3 with respect to the area percentage of the grains is ≤15％, and with respect to the number percentage of the grains is ≤10%; and the alloy casting piece comprises a main phase of the R2Fe14B type, an in-grain rare earth-enriched phase embedded within the grains, and a boundary rare earth-enriched phase distributed on the boundary of the grains, wherein the spacing of
